This invention relates generally to a system and method for key delivery using quantum cryptography, and, more particularly to a system and method for high data rate key delivery using quantum cryptography.
The transmission of information over a secure channel is an important component of many of the present commercial data exchanges. However, many of the present methods for providing secure information, usually referred to as crypto systems, rely on either the difficulty of computing certain characteristics or in securely distributing protected information, often referred to as a key, to each user. The logistics of the second approach are staggering and, therefore, most present systems use the first approach, often referred to as a public key system.
Another method of providing a secure channel between two parties, referred to as quantum cryptography (QC) or quantum key distribution (QKD), has been recently described. QKD relies on the fact that any measurements of quantities, such as phase or polarization of a photon, uniquely defines for the receiving and transmitting parties the state of the entity being measured.
Quantum cryptographic key distribution (QKD) systems transmit cryptographic key data encoded in the quantum states of individual optical photons. The benefits of such a system are that it allows secure transmission of key data over unsecured optical links with security guaranteed by the fundamental quantum properties of light rather than by computational complexity or barriers to interception. This is possible because single photons cannot be split into smaller pieces (intercepted or diverted photons simply won't arrive at the intended destination), nor can they be intercepted and consistently regenerated in identical states since their states cannot be fully characterized by single measurements, leading inevitably to errors in the states of the replacement photons. Practical systems for distribution of cryptographic keys using quantum cryptography protocols require transmission of single-photon optical signals through some medium, such as optical fiber or free space.
In a system described by C. H. Bennet and G. Brassard faint pulses of polarized light, assuring the light pulses are substantially single-photon pulses, are used to distribute key information via a low-attenuating (10–20 dB), non-depolarizing optical channel, called the “quantum channel”. By utilizing the “quantum channel”, two users can agree on a secret key in an impromptu manner, just before it is needed, but with provable security based on the uncertainty principle of quantum physics. To do so, the users may not exchange any material medium, but they do require a communication channel of a particular physical form, whose transmissions, owing to the uncertainty principle, cannot be eavesdropped on without disturbance.
In a system described by A. K., Ekert et al., a short-wavelength laser illuminates a suitably cut non-linear crystal. Two apertures select photon pair beams which are launched into single-mode fibers by lenses. Identical Mach-Zehnder interferometers are placed in the signal and idler arms of the apparatus. The interferometer outputs are viewed by signals and single-photon counting detectors.
In quantum cryptography, after the quantum transmission has been sent and received, the sender and receiver exchange further messages through a second channel, called the “public channel”, which may be of any physical form such as an optical, microwave, or radio channel. These messages, which need not be kept secret from the eavesdropper, allow the legitimate sender and receiver to assess the extent of the disturbance of the quantum transmission by eavesdropping by another and noise sources such as photomultiplier dark current, and, if the disturbance of the quantum transmission has not been too great, to distill from the sent and received versions of the quantum transmission a smaller body of random key information which with high probability is known to the sender and receiver but to no one else.
However, the bit rate obtainable in QKD systems based on present state-of-the-art technology is limited to approximately 1000 bits/second for short transmission distances and to a significantly lower bit rate for longer transmission distances. The gating technology controlling the bit rate of existing systems is the detector technology.
TABLE 1Estimation of QKD delivered key rate.Detector max clock rate:0.1–10MHz.Detector quantum efficiency:10dBOptical receiver loss:3dBFiber Span optical losses:10dB“μ < 0.1”1:10dBBasis guessing for BB84 protocol:3dBInformation leakage for Error Correction and<1dBPrivacy AmplificationTotal bit losses:37dB lossDelivered Quantum Key Rate:20–2000Hz
As shown in Table I, QKD systems operating at near infrared wavelengths are compatible with standard telecommunications optical fiber and typically have a maximum key generation rate in the range of 10 bits/sec to 1000 bits/sec, depending on the total optical fiber span length.
There is a need for enhanced bit-rate QKD architectures.